The present invention relates to a photographic camera and more particularly, to an exposure indicating device for use in a camera whose shutter speed is automatically controlled in response to the brightness of the object to be photographed or the scene brightness, which exposure indicating device informs the photographer that shutter speed to be effected automatically will be inside or outside of the available range.
In the cameras of the above described type, the shutter speeds are automatically controlled by means of exposure factors such as the brightness of the object to be photographed or the scene brightness, sensitivity of the film and the diaphragm aperture value or f number. Since the film sensitivity is determined by the film employed and the diaphragm aperture value is preselected by the photographer, the brightness of the object to be photographed is the main factor affecting the shutter speeds in acutal photography. In general such shutter speeds to be effected automatically may be broadly divided into three ranges, i.e., blur occurring range, e.g., the range of shutter speeds below a certain value, for example, one-thirtieth second, which will bring about blur in a picture taken with the camera supported by hand; upper or higher outside range which requires a high shutter speed not provided in the camera so that it is impossible to use in the camera; and an available range between the blur occurring and higher outside range. At such blur occurring shutter speed, either the use of camera stabilizing means such as a tripod is required or the employment of film having higher sensitivity or the adjustment of the f number is necessary, while at the upper outside shutter speed, a similar selection in the film sensitivity or the f number is required to bring the shutter speed to be effected automatically within the available range suitable for optimum exposure. To inform the photographer of such shutter speeds, an exposure indicating device for indicating any of the ranges into which the shutter speed fits is generally incorporated in the camera of the above described type.
In the conventional exposure indicating devices, for example, those disclosed in U.S. Pat. Nos. 3,712,194 and 3,742,827, it is so arranged that part of the depression stroke of a shutter release button of the camera is utilized for changing-over operational switches of a light measuring circuit in a manner as described hereinbelow.
Referring to FIG. 1 showing one example of known exposure indicating arrangements, the circuit generally comprises: a light measuring circuit which includes a photoelectric element as CdS cell Rp selectively connected in series to resistors R1 or R2 or the parallel connection of capacitor C1 and a switch S2 through a change-over switch S3 for generating, at a junction P1, an output representative of the brightness of the object to be photographed or the scene brightness when the photoelectric element Rp is connected to the resistor R1 or R2 and for forming a time constant circuit when the element Rp is connected to the capacitor C1; a reference voltage circuit including series-connected resistors R3 and R4 for generating reference voltage output at a junction P2; a switching circuit or differential amplifier A1 connected, at its input, to the output junctions P1 and P2 of the light measuring circuit and the reference voltage circuit; an indicating lamp L1 connected to the output of the differential amplifier A1 at the junction P3 and another indicating lamp L2 also connected to the output of the amplifier A1 through an inverting transistor Q1, the lamps L1 and L2 being further connected to the positive side of the power source E through a change-over switch S3'. The capacitor C1 forms a time constant circuit with the photoelectric element Rp for controlling an electromagnet Mg connected to the output of the amplifier A1 at the junction P3, which electromagnet Mg controls closing of the shutter (not shown) of the camera. The switch S2 is adapted to be opened in association with the opening of the shutter, while the switches S3 and S3' are actuated for changing-over of the indications in association with the depressing of the shutter release button.
In the above described conventional circuit arrangement, the movable switch contacts S3m and S3'm of the switches S3 and S3' are initially kept in contact with the respective stationary contacts S3a and S3'a, and upon depression of the shutter release button, the main switch S1 is closed, with the lamp L1 being connected to the power source E. At this stage, since the switch S3 is connected at the contact S3a, the potential at the junction P1 of the photoelectric element Rp and the resistor R1 is compared with the potential at the junction P2 of the resistors R3 and R4. The value of the resistor R1 is so determined that when scene brightness is so low as to provide shutter speed in the blur occurring range, the potential at the point P1 is lower than the potential at the point P2 of the reference voltage circuit thus generating an output from the differential amplifier A1 to energize the amp L1 for warning. Further depression of the shutter release button causes the movable contacts S3m and S3'm of the switches S3 and S3' to be switched over to the stationary contacts S3b and S3'b respectively, in which case, the potential at the point P1 is equal to the potential of the power source E divided by the photoelectric element Rp and the resistor R2. The resistor R2 has a resistance value such that the potential at the point P1 generated by series connection of photoelectric element Rp and the resistor R2 is higher than that at the point P2 when scene brightness is so high that the camera effect the shutter speed suitable for the brightness. Accordingly, when the scene brightness is a value to effect shutter speed in the available range, the potential at the point P1 is lower than that at the point P2 of the reference voltage circuit to produce output from the differential amplifier A1, but the lamp L1 is not lit since the movable contact S3'm of the switch S3' is connected to the stationary contact S3'b, while the lamp L2 is not energized either, through inversion of the output from the amplifier A1 by the transistor Q1. When the scene brightness is over the critical value which provides or effects the highest shutter speed available in the camera, no output is produced from the differential amplifier A1, since the potential at the point P1 exceeds that at the point P2. The transistor Q1 is supplied with base current through the electromagnet Mg and the resistor R5 to be rendered conducting, thus the lamp L2 is energized to warn that a shutter speed over the highest available is required.
The prior art exposure indicating device as described above, however, has various disadvantages in that the arrangement requires change-over switches for selectively conducting resistors, capacitor and lamps, and a mechanism for associating the shutter release button with the switches, and also in that the stroke for the depression of the shutter release button tends to be long, thus being unsuitable for incorporation into small sized cameras. Furthermore, some skill is necessary in depressing the shutter release button for checking the exposure conditions, since the prior art device is so arranged as to check the blur occurring shutter speed at the initial stage of depression of the shutter release button, with subsequent checking for the higher shutter speed upon further depression of the shutter release button, and then fully depression of the shutter release button for actual opening of the shutter.
Referring to FIG. 2, there has also been conventionally proposed another exposure indicating arrangement wherein the change-over switches such as the switches S3 and S3' in the circuit of FIG. 1 are dispensed with, and parallel-connected resistors r1 and r2 are connected between the photoelectric element Rp and the earth, and collector-emitter circuit of a transistor Q0 is connected in series to the resistor r2, while a rectangular wave oscillator PG is connected to the base of the transistor Q0 for driving the same transistor Q0. The junction P between the photoelectric element RP and parallel-connected resistors r1 and r2 is connected to a switching circuit A for energizing the lamp L by the output from the circuit A. By the above arrangement, independent connection of the resistor r1 to the photoelectric element Rp and connection of the parallel-connected resistors r1 and r2 to the element Rp are alternately established. When the scene brightness is so low as to provide shutter speed in the blur occurring range the switching circuit A produces output therefrom in both of the above cases for continuously energizing the indicating lamp L connected to the output terminal of the switching circuit A, while when the shutter speed is in the available range an output is produced from the switching circuit A only when transistor Q0 is conducting and for producing output from the switching circuit A only when the resistors r1 and r2 are in parallel with the indicating lamp L being alternately energized and de-energized. When the scene brightness is over the critical value which provides or effects the highest shutter speed available to the camera, no output is produced from the switching circuit A even during conduction of the transistor Q0, with the lamp L remaining de-energized. Although it does not require any change-over switches, the known arrangement as described above also has a serious disadvantage in that the adoption of the rectangular wave oscillator is particularly required for inclusion, thus resulting in complication of the circuit construction, with consequent high cost.